Constant Fraction Discriminator Research Articles

Constant-Fraction Discrimination/Boxcar Integrator for Plasma Source Time-of-Flight Mass Spectrometry

A constant-fraction discrimination (CFD) system has been combined with a boxcar integrator for detection in inductively coupled plasma/time-of-flight mass spectrometry. The discriminator provides gating logic for the boxcar integrator when an incoming ion signal occurs, but discriminates against electronic or background noise of lower amplitude. As a result, the combination can effectively reject noise and accumulate analyte signal, rather than relying on an averaging process to reduce noise levels. The signal-to-noise ratio is therefore enhanced in this operation compared with the conventional boxcar method. The dynamic range of the detection system is at least five orders of magnitude.

A two color mm-wave interferometer for the JET divertora)

An interferometer with compensation for vibration and large scale mechanical movements has been designed and built to measure the line integral electron density along three different lines of sight through the JET divertor plasma. Overcoming the effects of a long transmission path, having an estimated 65 dB loss, requires oversized waveguide transmission lines, sensitive heterodyne detection, low loss quasioptical circuits, and highly stable sources. The sources are frequency doubled, phase-locked, Gunn oscillators producing 15 mW at 130 GHz and 10 mW at 200 GHz. Waveguide Schottky mixer diodes generate reference and output signals at an IF of 10.7 MHz and the LO Gunn diodes are phase locked to the reference IF. Corrugated feedhorns and ellipsoidal mirrors are used for beam control and polarizing wire grids for beam splitting and recombination. To minimize unwanted, direct coupling of source power into the signal detectors, Brewster angle beam dumps and Faraday rotation isolators are used in the transmit and receive QO circuits, which in turn are separated, on opposite faces of a vertical plate. Martin–Pupplet polarizing interferometers are used to multiplex the two colors into a single coaligned, copolar output beam and to demultiplex the return beam. Constant fraction discriminators are used to optimize the accuracy of the phase detectors, which have sampling and recording rates of 1 MHz and a resolution of ∼7° (0.02 fringe).

A two color mm-wave interferometer for the JET divertor (abstract)a)

An interferometer with compensation for vibration and large scale mechanical movements has been designed and built to measure the line integral electron density along three different lines of sight through the JET divertor plasma. Overcoming the effects of a long transmission path, having an estimated 65 dB loss, requires oversized waveguide transmission lines, sensitive heterodyne detection, low loss quasioptical circuits, and highly stable sources. The sources are frequency doubled, phase-locked, Gunn oscillators producing 15 mW at 130 GHz and 10 mW at 200 GHz. Waveguide Schottky mixer diodes generate reference and output signals at an IF of 10.7 MHz and the LO Gunn diodes are phase locked to the reference IF. Corrugated feedhorns and ellipsoidal mirrors are used for beam control and polarizing wire grids for beam splitting and recombination. To minimize unwanted, direct coupling of source power into the signal detectors, Brewster angle beam dumps and Faraday rotation isolators are used in the transmit and receive QO circuits, which in turn are separated, on opposite faces of a vertical plate. Martin–Pupplet polarizing interferometers are used to multiplex the two colors into a single coaligned, copolar output beam and to demultiplex the return beam. Constant fraction discriminators are used to optimize the accuracy of the phase detectors, which have sampling and recording rates of 1 MHz and a resolution of ∼7° (0.02 fringe).

Stand-alone PC-based fast readout electronics and software for 2D-gaseous detectors

A novel readout system based on time signals processing and digitization for 2-D gaseous radiation detectors is presented. It is comprised of fast amplifiers, constant fraction discriminators, a dedicated 486 PC/AT-born time-to-digital converter and data acquisition boards and software. Its software package runs under Windows 3.1. The system allows for fast real-time data acquisition as well as for on-line and off-line data processing and image analysis. At event rates reaching 3 kHz this readout system provides an efficient and convenient tool for laboratory tests. It can operate at higher rates, exceeding 0.5 MHz under DOS. The entire system is stand-alone and does not require any additional electronics. It can operate under both external and self-triggering modes. The novel system is versatile and may be applied for the readout of a large variety of gaseous imaging detectors. Images acquired by X-ray and β imaging avalanche gas detectors equipped with this electronic system and using the software package are presented.

Performance of non-delay-line constant-fraction discriminator timing circuits

The delay line in the constant-fraction discriminator (CFD) can be replaced with circuit networks to permit monolithic fabrication. The previously reported non-delay-line CFD utilizing a single-pole highpass filter is reviewed, followed by a presentation of non-delay-line circuits utilizing Gaussian lowpass filters. Circuit response for non-delay-line CFDs utilizing single-pole highpass or single-pole lowpass filters is shown to be equivalent. However, timing-signal underdrive and zero-crossing slope are increased and timing jitter reduced when higher-order lowpass filters are used. Comparisons of non-delay-line and delay-line CFD circuit timing performance are presented for lowpass-filtered step inputs representative of scintillation detector signals. Timing resolution of non-delay-line and delay-line CFDs is compared for BGO/photomultiplier scintillation detector applications. Monte Carlo simulations indicate that comparable timing resolution is available with proper selection of non-delay-line CFD filter time-constants. A fully-monolithic CMOS, non-delay-line CFD was fabricated to experimentally validate timing resolution. The measured timing resolution of 3.26 ns FWHM and 6.5 ns FWTM is in good agreement with measured resolution of 3.30 ns FWHM and 6.4 ns FWTM for a standard delay-line CFD circuit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fast multilayer fission chamber with 239Pu

For the investigation of neutron induced fission of heavy nuclei in the resonance energy range the fast multilayer ionization fission chamber with a 239Pu content of 1.6 g was constructed. The chamber is compact and a minimum of material has been used for its construction. The chamber is divided into 19 sections containing no more than 100 mg of 239Pu in a section whose intrinsic capacity is less than 100 pF. By using fast preamplifiers and constant fraction discriminators together with the combined method of amplitude and pulse length discrimination the background due to α-particles is suppressed and a less perturbed pulse height distribution is obtained. The absolute fission fragments detection efficiency of the chamber is (60±8)%. Its time resolution does not exceed 2.6 ns.

Optimization of scintillation-detector timing systems using Monte Carlo analysis

Monte Carlo analysis is used to model statistical noise associated with scintillation-detector photoelectron emissions and photomultiplier tube operation. Additionally, the impulse response of a photomultiplier tube, front-end amplifier, and constant-fraction discriminator (CFD) is modeled so the effects of front-end bandwidth and constant-fraction delay and fraction can be evaluated for timing-system optimizations. Such timing-system analysis is useful for detectors having low photoelectron-emission rates, including bismuth germanate (BGO) scintillation detectors used in positron emission tomography (PET) systems. Monte Carlo timing resolution for a BGO/photomultiplier scintillation detector, CFD timing system is presented as a function of constant-fraction delay for 511-keV coincident gamma rays in the presence of Compton scatter. Monte Carlo results are in good agreement with measured results when a tri-exponential BGO scintillation model is used. Monte Carlo simulation is extended to include CFD energy-discrimination performance. Monte Carlo energy-discrimination performance is experimentally verified along with timing performance (Monte Carlo timing resolution of 3.22 ns FWHM versus measured resolution of 3.30 ns FWHM) for a front-end rise time of 10 ns (10-90%), CFD delay of 8 ns, and CFD fraction of 20%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The sixteen-channel APEX constant fraction discriminator

A sixteen-channel constant fraction discriminator (CFD) has been constructed for the ATLAS Positron EXperiment (APEX). An integrated circuit, recently introduced commercially, contains all the electronic building blocks required to construct a CFD and makes it possible to fit sixteen CFD channels into a single-width CAMAC module. An important new feature is the time-to-charge converter that is part of each CFD channel. Its calibration constant is controlled via CAMAC. The time-to-charge converter allows the use of low-cost charge-sensitive analog-to-digital converters for timing measurements.

A compact TDPAC spectrometer

A fast-fast TDPAC spectrometer consisting of four BaF 2 detectors, a simple constant fraction and routing interface module, a TAC, an ADC and a personal computer for data administration has been developed. Four double-window constant fraction discriminators, allowing simultaneous timing- and energy discrimination, a routing unit and an interface to the personal computer are integrated in the double-NIM module. The fast component in the scintillation light of BaF 2 make such an exceptionally simple TDPAC spectrometer possible, which has a time resolution of 200(10) ps FWHM for the 1173–1333 keV γ-cascade of 60Co.

A fast constant-fraction discriminator for [formula omitted] separation with a lead/scintillating-fiber calorimeter

In a lead/scintillating-fiber calorimeter with fibers oriented in the direction of the incident particles, the different longitudinal development of the showers generated by electrons and pions results in a different time structure of the signals. We have designed a constant-fraction discriminator, which allows the achievement, on-line, of the electron/pion ( e π ) separation needed for trigger purposes. The fast signals from the calorimeter, of 10–20 ns duration, demand a circuit able to operate in the high frequency domain. We tested the performance of the circuit with signals produced by a prototype calorimeter constructed by the LAA-SPACAL collaboration at CERN. The circuit allows triggering on electrons with an efficiency higher than 99% with about 6% of the pions faking electrons in the dynamic range from 0.2 V to more than 2 V. The decision time is shorter than 60 ns and the typical time resolution is better than 100 ps.

Electronics for the Huygens Vat 4π detection system

This article describes a simple, low cost bus system used for digital control of parameters like, for instance gain and trigger level, in analog nuclear front-end electronics. Several modules used in this system, such as shaping amplifiers and constant fraction discriminators, are also described.

A simple software technique for resolution improvement in time spectroscopy measurements

Time spectroscopy measurements, such as in neutron time-of-flight experiments, are limited in resolution by random walk of the pick-off time marking the arrival of a timing pulse due to variations in the pulse amplitude and shape, as well as time jitter introduced by additive noise. Hardware techniques, such as the constant fraction discrimination, are employed to improve the pick-off accuracy, but residual variations remain since improvement in the performance with walk is usually obtained at the expense of increased jitter. The author describes a simple software approach for further improvement of the timing resolution through the on-line averaging of a number of successive time measurements. Results are presented for the application of the technique to simulation data as well as to experimental data pertaining to resolution measurements on an NE213 scintillation detector. An account of possible applications for the proposed technique is given.

Development of a monolithic constant fraction discriminator

A constant fraction discriminator was developed with a bipolar application-specific IC (ASIC) technology from NEC without a delay. It is called the differentiation constant fraction discriminator (DCFD). Prototype chips were examined for their integrity and performance. This technology has significant advantages over others currently in use for applications in high-density integration, and particularly for some large-scale experiments such as the B-factory and SSC. The performance of this differentiated constant fraction integrated circuit was tested by a trapezoidal input signal. The function of the DCFD IC was exactly as expected, and the full width of the time walk was 1 ns or less.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A precision timing discriminator for high density detector systems

Most high-resolution time measurement techniques require discriminators that accurately mark the time arrival of events regardless of their intensity. Constant fraction discriminators or zero-crossing discriminators are generally used. Most are sensitive to the shape of event waveforms and may require cumbersome delay lines. The authors describe a zero crossing discriminator which accurately detects the maxima and minima of a measured waveform. Basically, it consists of a fast voltage comparator and a single integrating time constant, RC. A time walk of 100 ps in an amplitude range exceeding 100:1 has been achieved for waveforms from 1.5 ns to 15 ns full width at half maximum (FWHM). An arming level discriminator is added to eliminate triggering by noise. Easily implemented in either monolithic of hybrid technology, the circuit is suitable for large multichannel detector systems where size and power dissipation are crucial. Circuit diagrams and typical measured data are also presented. >

Timing performance of the Thorn EMI 9127 and 9128 photomultipliers

Three recently designed Thorn EMI photomultipliers (9127, 9128 and D192) have been evaluated in this work. They are all 28 mm in diameter, 11-stage devices. The 9127 has a flat photocathode and the 9128 and D192 have plano-concave windows to minimise the variation in distance between any point on the photocathode and the first dynode. In addition the D192 has a RbTe photocathode which makes it sensitive only to UV wavelengths in the range from 190 to 280 nm. The best single counter timing resolutions, uncorrected for pulse height effects, obtained were 150, 160 and 220 ps for the 9127, 9128 and D192 respectively for detection of coincident 511 keV photons. Correcting for the time walk of the constant fraction discriminators, we achieved a counter resolution of 120 ps with the 9128. The electron transit time variation across the photocathode was measured and used to explain the relative timing performance of the tubes with different dynode chains and scintillator geometries. The leading edge jitter of the output pulse has been measured in an independent way using a mode locked picosecond-pulse dye laser to be 125 ps.

Amplitude and risetime compensated time pick-off circuit based on extrapolated leading edge timing technique

A time pick-off circuit for use with large high-purity germanium (HPGe) detectors and its performance are described. It uses two discriminators to extrapolate the time origin of the input pulse thereby reducing the walk due to variation in amplitude and risetime. In a commonly used constant fraction discriminator (CFD), the cable decay used for shaping, arming discriminator bias and other circuit parameters are to be critically adjusted to achieve a compensation for the walk. In the circuit described cable delay and arming discriminator are not required and the adjustment of the discriminator biases is simple. The performance of this simple circuit has been found to be comparable to that of a commercially available complex CFD unit (EG&G ORTEC model 473A). A typical delayed coincidence curve pertaining to the 122 keV level of 152Sm is also presented to demonstrate the effectiveness of the circuit in measuring short half-life (1.4 ns) using a 127 cm3 HPGe detector and a BaF2 scintillator combination. This circuit is thus a low-cost and simple alternative to a CFD unit.

A monolithic, 2 mu m CMOS constant-fraction discriminator for moderate time resolution systems

A monolithic, 2 mu m CMOS constant-fraction discriminator (CFD) was developed for use with moderate time resolution (about 4 ns FWHM) scintillation detectors such as BGO. The circuit was fabricated through the metal-oxide-semiconductor implementation service (MOSIS) using a double-metal, double-polysilicon digital process. The measured CFD walk is 1.4 ns total for input signals of -200 to -2000 mV with risetimes of 20 ns. The measured CFD noise jitter is 70 ps FWHM for -200 mV, 20 ns risetime signals. Measured timing performance of the CFD circuit with a BGO scintillation detector shows good performance for detectors with timing resolutions in the 4 ns FWHM range. A general-purpose comparator, included on the same die as the CFD for separate evaluation, has measured response times of 32 ns for a 50 mV overdrive input and 23 ns for a 500 mV overdrive input. Both the CFD and the general-purpose comparator have application in large multichannel systems, including positron-emission tomography (PET), where circuit size and power consumption are limited for each system channel. >

Minimization of discrimination jitter in time-correlated single photon counting

Instrument function widths in time-correlated single photon counting are minimized by adapting a commercial constant-fraction discriminator for essentially jitter-free processing of anode pulses from a Hamamatsu R1564U microchannel plate photomultiplier. This modification yields an instrument function with 47 ps FWHM, limited chiefly by photomultiplier transit-time spread.

Design and performance evaluation of a miniaturized particle desorption mass spectrometer

A miniaturized particle desorption mass spectrometer PDMS has been designed, assembled and tested. A theoretical model has been used to analyze the transmission efficiency and to predict the mass resolution performance. The setup consisted of (a) a 50 μCi 252Cf source placed behind a thin target (transmission mode), (b) a 15 cm flight tube of 2 cm diameter and (c) microchannel plate start and stop detectors. The detector signals were processed via constant fraction discriminators through a time-to-amplitude converter and stored in a multichannel analyzer. Mass resolution was dependent upon both mass and sample bias. For a mass of 133 amu and sample bias of 1 kV, the mass resolution (mΔm) was 130. Desorbed ions were focused by means of an electrostatic particle guide. The effect of the initial ion energy and angular distributions on the optical transmission efficiency of this guide has been analyzed. The overall instrument transmission efficiency was approximately 40%. The detection limit for Li has been estimated at 0.5 μmol. The applicability of PDMS to environmental samples was demonstrated with the analysis of pesticides.

A new idea of automatic walk stabilization for constant fraction discriminator

The idea of automatic walk stabilization for constant fraction discriminator based on compensated base line restoration is presented and tested. Comparison of this idea to gated base line restoration and other principles is discussed.